In the case of processing or sliding against a steel sheet with the surface thereof being made of a soft metal such as Zn, etc., that is, for example, a Zn-coated steel sheet, a problem can occur in that the soft metal adheres to the surface of a member (for example, a die, etc.) in contact with the soft metal to be further deposited on the surface of the member, thereby impairing the surface quality of a workpiece.
More specifically, there exists, for example, the following problem with a hot pressing method, as hot forming. The hot pressing (also called as “die-quench”) method represents a technique whereby, upon a steel sheet (blank) being heated to a temperature (normally, 800 to 900° C.) in an austenite region to be quenched in a water-cooled die, the steel sheet is formed into the desired shape of a part. Process-steps from the heating of a steel sheet up to press working are carried out in the atmosphere from the view point of a cost, and therefore, for the purpose of controlling scale-formation due to oxidation of a steel sheet, a plated steel sheet with a plating layer composed of, primarily Al or Zn, formed on the surface thereof, is used for the steel sheet. However, in the case of using the plated steel sheet, that is, the zinc (Zn)-coated steel sheet in particular, Zn is adhered to a die for pressing along with an increase in the number of shots, thereby reaching a stage in which the die undergoes a change in shape in extreme cases, whereupon there arises a problem with a product shape, and the surface quality of a steel sheet as formed.
In general, a die for use in the hot pressing, and so forth is formed after coating the surface of the die with a ceramic film of TiN, etc., as countermeasures against abrasion wear. Even in this case, however, anti-adhesion property to the soft metal is far from sufficient.